Repeated calls to RequestHandler.set_cookie with the same name now
overwrite the previous cookie instead of producing additional copies.

tornado.web.OutputTransform.transform_first_chunk now takes and returns
a status code in addition to the headers and chunk. This is a
backwards-incompatible change to an interface that was never technically
private, but was not included in the documentation and does not appear
to have been used outside Tornado itself.

Fixed a bug on python versions before 2.6.5 when URLSpec regexes
are constructed from unicode strings and keyword arguments are extracted.